A routine inspection revealed significant corrosion of the prestressing strand on a concrete road bridge built in 1966 to a standard design used in about 117 State Highway bridges in New Zealand. To identify the cause of the deterioration and how many bridges of this design might be affected, the conditions of 29 similar bridges on New Zealand State Highways were evaluated by site investigation. The research, carried out in 2005–2006, found that although the concrete quality in the bridge beams was generally good, the combination of cover depths less than 25 mm and exposure to salt spray had increased the likelihood of corrosion in bridges of this design in the B2 (coastal frontage) exposure zone. Bridges in the B1 (coastal perimeter) and A2 (inland) zones are less likely to be affected, although the concrete in some of the beams contained chlorides added during construction. The risk associated with prestressing corrosion in this beam design is higher than in current designs because the prestressing strand is poorly confined and the cover depth is low. Bridges of this design in the B2 zone will probably need some form of intervention to remain serviceable for a 100-year service life.